It is known that sulphur or selenium react with aromatic compounds according to the following equation: ##STR6##
The reaction is generally catalyzed by a halogen (bromine, iodine), an aluminium halide (chloride, bromide, iodide), a ferric halide (chloride, bromide), zinc chloride or antimony chloride, as has been described, for example,
in French Patents 1,029,987 or 1,058,936, or in U.S. Pat. Nos. 2,415,363 or 2,433,658.
According to the known processes, aromatic sulphides, and more particularly phenothiazines, can be obtained by reacting two moles of sulphur per mole of diphenylamine in the presence of a catalyst, the quantity of which is generally less than 3% by weight. The reaction takes place at a temperature above 120.degree. C. The reaction kinetics increase with increasing temperature and it can be advantageous to work at a temperature of between 180.degree. and 200.degree. C., since a temperature greater than 200.degree. C. leads to the formation of tars and undesirable by-products. In addition, implementation of the known processes leads to the formation of a stoichiometric quantity of hydrogen sulphide (H.sub.2 S).
Generally, during implementation of known processes, the reaction speed reduces considerably when the level of conversion of the diphenylamine reaches 90%. To obtain a complete reaction, it is therefore necessary to raise the temperature and to remove the hydrogen sulphide by means, for example, of an inert gas such as nitrogen, argon or carbon dioxide. Sometimes the slowing in the reaction speed can be due to a loss of catalyst, which is entrained in the effluent. Thus, when iodine, which is the catalyst which is generally used in this type of reaction, is used, there is a loss in the form of hydriodic acid. It is therefore necessary to increase the quantity of catalyst or even to add the catalyst continuously.
However, the principal drawback of the known processes lies in the production of hydrogen sulphide. In addition to the fact that the formation of the hydrogen sulphide consumes half of the sulphur used, it is necessary to remove it which entails, for industrial hygiene and environmental protection reasons, the use of costly industrial apparatus.
It has been proposed to replace the sulphur by a sulphur-containing compound such as sulphur chloride, sulphur dichloride, sodium thiosulphate, antimony or arsenic sulphide, an alkaline polysulphide, or thionyl chloride, which lead to a smaller formation of hydrogen sulphide. However, yields are then generally mediocre, and the products obtained require long and difficult purification.